This invention relates to an imaging optical sensor, and, more particularly, to a blur film assembly utilized in the calibration of an infrared imaging optical sensor.
In one commonly encountered configuration, an imaging infrared (IR) optical sensor (i.e., a focal plane array or FPA detector) consists of a large number, typically thousands or tens of thousands, of individual electro-optic detector elements, which are positioned at the focal plane of the optical system. The detector elements view a scene through an appropriate optical path. The materials and/or construction of the detector elements may be selected to be sensitive to different wavelength ranges (wavebands) of electromagnetic radiation, including, for example, infrared energy. The detector elements are arranged in a planar array, with each detector element providing one pixel of an image. The outputs of the detector elements are digitally processed to form a recreation of the image which may be further analyzed.
Ideally, all of the detector elements would respond identically to incident energy, with the signal output of each detector element identically proportional to the incident energy. In practice within the limitations of current technology, however, each of the different detector elements may be expected to respond slightly differently. Changes in the pixel responses may also develop in the detector array with time or with use. These differences may be evident as gain or zero-point offsets, nonlinearity, or other types of departures from the ideal identical response. As a result of such departures, if a perfectly uniform, infrared input scene were presented to the FPA detector, the detector output would not be perfectly uniform. A variety of techniques are known for both reducing the departure from the ideal in the mass-produced detector elements during production, and also for compensating for non-ideal responses which develop during service. The present invention is concerned with the electronic compensation for non-ideal responses of detector elements which are present in the detector as initially manufactured or which develop in the detector over time and use.
If a scene of uniform energy is viewed by many detector elements during a calibration period and their electrical outputs are different, compensation may be made by adjusting either the zero point and/or the gain of the output of each detector element to achieve an identical output response. This calibration adjustment to the zero point and/or gain is maintained when the scene of interest is later viewed. The overall output radiance of the uniform energy scene during calibration should be approximately that encountered when the scene of interest to the application is viewed, because the departures from non-ideal responses are intensity dependent. In a laboratory setting, then, calibration of the infrared detector elements may be accomplished by providing a uniform calibration energy field separate from the scene, detecting the average radiance of the scene, adjusting the output radiance of the calibration energy source to that of the average radiance of the scene, viewing the calibration energy source with the individual detector elements, and adjusting the zero point and/or gain of the individual detector elements which depart from their ideal electrical output.
In some sensor applications, it is not practical to have available a separate uniform energy source which may be viewed by the detector array. In a missile seeker, for example, there is physically not sufficient space for a separate calibration energy source and the optical path required to direct the output of the energy source onto the detector array during a calibration period.
A blurring technique has been previously developed for application in such situations. During calibration, the view of the scene is controllably blurred over a group of detector elements. In blurring, the scene energy containing relatively high spatial energy frequency components is spread or diffused to produce a blurred image having a more uniform energy level. Any measured high spatial frequency that remains is thus due to the detector elements' non-uniform responses and is driven to zero by electronic compensation. The average radiance of the blurred image approximates that of the scene, and therefore best optimizes the system for the operating conditions. Calibration proceeds as described above.
The controlled blurring of the scene presents a challenge, and a variety of techniques have been used. For example, with one known approach, the scene is viewed by the detector array through two optically transparent elements of different optical path lengths (thickness and/or refractive index). One element is selected such that the image is focused onto the focal plane of the detector array, and the other element (used for calibration) is selected such that the focus is longitudinally displaced from the focal plane of the detector array. While operable, in practice it has been found for infrared sensors that the blurring is insufficient, and that relatively high spatial frequency features of the scene may still be discerned, which prevents accurate calibration of the FPA pixel response.
There is a need for an improved approach to the calibration in service of focal plane array detectors, particularly in systems with restrictive space constraints such as infrared missile seekers. The present invention fulfills this need, and further provides related advantages.